I use Webjay alot for streaming music. The site allows you to define playlists of (legally) downloadable MP3s. The playlists you create are connected to others that share the same songs. The resulting network is a great way to discover new music - there is a better chance that I'll like a song that somebody else likes if I know that we already share tastes in music.
So I get exposed to lots of great new music and artists. Oftentimes, as I listen, I think 'X would like that' where X is some friend of mine. As it stands now, I manually send an email to X with a link to the song in question. This is less than ideal as there is no record of the connection for future reference and X has to keep track of these sorts of 'invitations'.
I think there is value in the idea of people defining lists (songs, bookmarks, radio stations, geolocations, etc) FOR others rather than themselves.
A system which exposes people to new things (e.g. music, sites, etc) would be valuable - that's the problem with sites like iTunes - you have this wealth of great music that you just know has songs you will love, but but don't know how to get to them.
What your friends think would be interesting "to you" would be a valuable entry point (far more than some untargetted 'Whats popular')